The steel industry uses synthetic slags in secondary steel making. The roles of synthetic slags are varied and include inclusion engineering, desulphurisation and heat transfer during ladle re-heating.
Different synthetic slag compositions are used for different purposes. Alumina (Al2O3), lime (CaO) and sometimes MgO and other minor components are examples of components of such synthetic slag compositions. The nature and quantity of such components of the ultimate synthetic slag compositions used in steel making are determined by the steel maker and are not in itself aspects of the present invention. The present invention is concerned with the alumina component to be introduced, if required, into such synthetic slag compositions. Alumina is added to lower the melting point of the synthetic slag thus making it more fluid at working temperature. An alumina containing product which is to be used as a component of a flux in such steel making process needs to be of a particulate nature having a size distribution of larger than about 3 mm and smaller than about 60 mm in cross dimension. This constraint is general because of the nature of furnace designs and dynamics which precludes the use of finer or powdery products.
This invention relates to the use of a waste product, namely powdery aluminium dross derived from aluminium smelters, as a raw material for producing an agglomerated product containing alumina which may be used as a component of the slags of the type in issue. Powdery alumina dross is the final waste after all recoverable aluminium metal has been removed from a dross originating from an aluminium smelting furnace. In the recovery of residual aluminium metal from aluminium dross originating from an aluminium furnace a milling process is conventionally used to reduce the hardened dross to a powder. The powdered dross is then passed through screens and filters and aluminium metal is recovered from the screens and filters while the product passing through the screens or filters, known as powdery aluminium dross, is treated as waste material. Large tonnages of this waste material end up in waste dumps. Some of the dross so produced during recovery of aluminium from dross is however also offered for sale by aluminium recovery operations. The dross so offered for sale is typically offered in different grades of fineness originating from the different screening and filtering operations. Thus so-called “coarse dross” typically has a particle size distribution of less than 1mm in maximum cross dimension. “Fine dross” or “Filter dross” having a particle size of typically less than 200 μm is another available product.
The major component of powdery aluminium dross is alumina (Al2O3) which typically account for between 30% and 60% of the mass of the dross. The powdery aluminium dross also typically contains aluminium metal. Despite the efforts to recover as much as possible of the aluminium content of the powdered slag, it may typically contain between about 1% and about 10% aluminium metal (Alm) by mass. The powdered aluminium dross further typically contains aluminium values in the form of aluminium nitride (AlN) which is typically present in the dross in a quantity accounting for about 12% to about 30% of the mass of the dross. It has been established that the finer dross products generally have lower Al(m) content than the coarser dross products.
Other components which are typically present in powdery aluminium dross, and the quantities in which they are present, if present, are as follows:    Fe2O3—up to 1.0%    MnO—up to 0.2%    Cr2O3—up to 0.02%    V2O5—up to 0.15%    TiO2—up to 0.2%    CaO—up to 1.5%    K2O—up to 0.7%    S—up to 0.2%    P2O5—up to 0.3%    SiO2—up to 5%    MgO—up to 5%
In the applicant's patent application WO 2009/004565 it was disclosed that the alumina present in powdery aluminium dross may be utilised as a source of alumina for a desulphurising flux in the steel making industry by forming the powdery aluminium dross into an agglomerated product in the form of pellets or briquettes with the aid of a binder material, namely cement, and in particular a high alumina cement. In the method disclosed in WO 2009/004565 the powdery dross is mixed with the cement and water and agglomerated into pellets or briquettes which were spread open to the atmosphere and allowed to dry, and then calcined by a self-sustaining combustion of the pellets or briquettes in a vertical kiln.
The applicant has now found a method whereby an agglomerated alumina containing product may be produced from powdery aluminium dross without the need to introduce a binder, thereby avoiding the introduction of extraneous elements and components which elements or components are not required to be present in the flux composition when the alumina containing product is used as a component of a synthetic slag in steel making.